This application relates generally to gas turbine engines and, more particularly, to a flowpath through a rotor assembly.
A gas turbine engine typically includes at least one rotor assembly including a plurality of rotor blades extending radially outwardly from a plurality of platforms that circumferentially bridge around a rotor disk. The rotor blades are attached to the platforms and root fillets extend between the rotor blades and platforms. An outer surface of the platforms typically defines a radially inner flowpath surface for air flowing through the rotor assembly. Centrifugal forces generated by the rotating blades are carried by portions of the platforms below the rotor blades. The centrifugal forces generate circumferential rim stress concentration between the platform and the blades.
Additionally, a thermal gradient between the platform and the rotor disk during transient operations generates thermal stresses which may adversely impact a low cycle fatigue life of the rotor assembly. In addition, because the platform is exposed directly to the flowpath air, thermal gradients and rim stress concentrations may be increased. Furthermore, as the rotor blades rotate, blade roots may generate local forces that may further increase the rim stress concentration.
To reduce the effects of circumferential rim stress concentration, additional material is attached to each root fillet to increase a radius of the root fillet. However, because the root fillets are exposed to the flowpath air, the additional material attached to the root fillets may be detrimental to flow performance.
Other known rotor assemblies include a plurality of indentations extending between adjacent rotor blades over an axial portion of the platforms between the platform leading and trailing edges. The indentations are defined and formed as integral compound features in combination with the root fillets and rotor blades. Typically such indentations are formed using an electro-chemical machining, ECM, process. Because of dimensional control limitations that may be inherent with the ECM process, surface irregularities may be unavoidably produced. Such surface irregularities may produce stress radii on the platform which may result in increased surface stress concentrations. As a result, the surface irregularities then are milled with hand bench operations. Such hand bench operations increase production costs for the rotor assembly. Furthermore, because such indentations extend to the platform trailing edge, a forward facing step is created for an adjacent downstream stator stage. Such steps may be detrimental to flow performance.
In an exemplary embodiment, a rotor assembly includes a plurality of indentations for facilitating a reduction in circumferential rim stress during engine operations. More specifically, in the exemplary embodiment, the rotor assembly includes a rotor including a plurality of rotor blades and a radially outer platform. The rotor blades are attached to the platform and extend radially outward from the platform. The platforms are circumferentially attached to a rotor disk. A root fillet provides support to rotor blade/platform interfaces and extends circumferentially around each rotor blade/platform interface between the rotor blade and platform. The platform includes an outer surface having a plurality of indentations that extend between adjacent rotor blades. Each indentation extends from a leading edge of the platform to a trailing edge of the platform. Each indentation is tapered to terminate at the platform trailing edge with a depth that is approximately equal zero.
During operation, as the rotor blades rotate, centrifugal loads generated by the blades are carried by portions of the platforms below each rotor blade. As air flows between adjacent rotor blades, the platform indentations facilitate a reduction in thermal gradients that may develop between the platform and rotor disk, thus, reducing thermal stresses that could impact a low cycle fatigue life (LCF) of the rotor assembly in comparison to other rotor assemblies. The indentations provide stress shielding and reduce stress concentrations by interrupting circumferential stresses below the rotor blade root fillets. Because a radius of each indentation is larger than a radius of each root fillet, a lower stress concentration is generated in the circumferential stress field and less circumferential rim stress concentration is generated between the platform and the rotor blades in comparison to other rotor assemblies. As a result, the rotor assembly facilitates high efficiency operation and reducing circumferential rim stress concentration.